The present invention relates to liftcranes and in particular to heavy duty liftcranes that use a hoist block sheave arrangement.
Liftcranes are used for a variety of lifting tasks. When liftcranes are used for lifting very heavy loads one arrangement that has been devised is to employ a hoist block sheave arrangement. A hoist block sheave arrangement uses upper and lower block halves suspended from the end of the liftcrane boom. Each of the block halves includes a plurality of corresponding sheaves. The lower block half may also include a hook or other similar device to which the load can be attached. The upper and lower block halves are connected by hoist rope or cable that is reeved through the corresponding sheaves of each block half.
The purpose of the hoist block sheave arrangement is twofold. First, the multiple sheaves connecting the upper and lower block halves provide a mechanical advantage as an arrangement of multiple pulleys. Secondly, lifting can be accomplished using two drum hoists instead of one. This latter advantage can be obtained because a single length of rope is reeved through the sheaves of the hoist block and each end of the rope is wound around a separate hoist drum on the liftcrane. Thus, the load can be lifted using not only the mechanical advantage of the multiple pulleys, but also with the lifting power of two hoist drums. Examples of liftcranes that use hoist block sheave arrangements include Models 4000, 4100, and 36 ft. platform Ringers manufactured by the Manitowoc Engineering Co. of Manitowoc, Wisconsin. Some of these liftcranes can lift loads of 800 to 1400 tons or more.
When a hoist block sheave arrangement is used in the manner as explained above, a relatively great length of rope is required, e.g. 4500 feet. This is because a single rope is reeved through the multiple hoist block sheaves and both ends of the rope are run all the way back to the two hoist drums. Using a single rope of this great length can present disadvantages. For example, it is cumbersome to dismantle the hoist block sheave arrangement in case the liftcrane has to be moved. Also, since only a single rope of great length is used, neither the front nor the rear drum is typically large enough to hold the entire length of rope. Thus, the rope must be removed entirely from the liftcrane and wound onto a separate spool. Then, in order to use the liftcrane again, the rope must be wound off the spool and reeved through the hook block sheaves and boom and around both the front and rear drums. Thus, additional time and effort must be expended in order to take advantage of the hoist block sheave arrangement.
According to an aspect of the present invention, there is provided an improved method and system for a liftcrane in which a load is lifted through the combined action of first and second hoist drums. The method and system use two ropes. A first rope-is wound on one hoist drum and a second rope wound on the second hoist drum. The ropes extend over a boom and the ends of the ropes opposite the hoist drums are linked together so that tension can be transmitted between them. The load is lifted by a hook carried by the linked ropes. If the take up speed of one of the ropes exceeds the take up speed of the other rope, the linked ends of the ropes will shift. This condition is detected and the operation of at least one of the first and second hoist drums is modified to adjust the take up rates of the two ropes into balance. This system is advantageously used with a hoist block sheave arrangement.